This invention relates to an electrophotographic printing machine, and more particularly to a photoreceptor belt and copy paper support arrangement that reduces transfer deletions.
This invention is particularly applicable to high speed xerographic copy reproduction machines. These machines conventionally include an endless xerographic photoreceptor belt that travels through a closed loop path from one station to another. For example, the belt is corona charged at a first station and exposed to an image at a second station. The image is developed at a third station and then proceeds to a fourth station generally known as a transfer station before it is precleaned and the remaining toner removed from the belt for use in the process again.
As is well known in the art, copy sheets are fed from one or more trays and advanced through a copy sheet transport path for operative engagement with the photoreceptor belt at the transfer station. This invention also contemplates receiving paper from a duplex tray where a copy sheet has already passed through the transfer station for imaging on one side, temporarily stored in a duplex tray, and subsequently fed through the transfer station a second time in an inverted state so that the opposite side of the copy sheet is imaged.
In reprographic systems of this type, transfer deletion is often a problem. That is, insufficient contact between the copy sheet and the photoreceptor belt results in incomplete or insufficient transfer of the image from the belt to the sheet. As is known in the art, this image transfer occurs at a transfer station where the paper is charged to receive the image from the photoreceptor belt. Intimate contact between the copy sheet and the photoreceptor belt at the transfer station allows the toner to pass from the photoreceptor to the copy sheet.
Insufficient contact between the photoreceptor belt and the copy sheet can result for various reasons. A primary cause of insufficient contact and transfer deletion is associated with imperfections in the copy sheet, wrinkles or cockles in duplex sheets that have been printed on one side, or puckers and pockets associated with the copy sheet or photoreceptor belt. Non-ideal transfer of xerographic images from belt photoreceptors to the copy paper are, therefore, often due to paper damage. Heating and cooling associated with the fusing process, where the image previously transferred to the copy paper is heated for bonding with the paper, may result in non-uniform moisture content in the copy paper. This, of course, is associated with duplex copying where the copy paper passes through the process twice to receive images on opposite sides of the paper. Damage may also be associated with the paper handling system, such as the drive and cross feed rolls. Again, the paper can become wrinkled or cockled thereby resulting in less than desirable transfer of the image.
At one time the use of substantially planar transfer stations was encouraged for different reasons. For example, heavier grade paper has a tendency to separate from the photoreceptor belt, particularly at the trailing edge, resulting in insufficient transfer at that area. In an effort to address this perceived problem, planar transfer zones were encouraged to assure that the entire copy paper received the image from the belt. Nevertheless, it has been observed that more severe transfer deletion problems are associated with planar or flat transfer stations.
Accordingly, it has been deemed desirable to overcome transfer deletion problems, particularly those associated with copy sheet damage.